It is known to sort objects within a bulk of objects, such as grains, by means of a rotating cylinder or drum, which cylinder has pockets on the inside. This cylinder is rotating around a substantially horizontal axis, while being aligned with its longitudinal central axis coinciding with said horizontal axis. The objects, such as granules, are feed into one end of the cylinder, and as the cylinder rotates the granules will be lifted as they are captured in the pockets. The pockets are adapted in size and dimensions for receiving one object each. In the bottom of each pocket an opening is provided into the outside surface of the drum, such that for example light may be sent outside the drum, through the openings, onto the objects, and being detected on the inside of the drum or reflected to be detected on the outside of the drum, or vice versa. In this way the object in the respective pocket may be illuminated with light, and reflection or transmission spectra may be obtained. From this spectra, characteristics of said objects may be obtained, which may be used to sort or fractionize said bulk of objects based on said characteristics. One or several collectors may then be placed in the vicinity of the drum, to receive—after characterization—a specified fraction based on impulses from a detector. A drum of this kind, and a machine comprising such drum, is disclosed in WO 2004/060585.
A problem associated with such a drum is that the opening may be occluded by the objects received therein, if the objects get stuck in the opening. Then this pocket will be useless for the rest of the fractionizing procedure, since the object being stuck not will be expelled into its corresponding through and no new object can enter the pocket during the next revolution of the drum. As the fractionizing procedure continues the problem increases as more and more openings get occluded by objects.